Cp2Mg (cyclopentadienyl magnesium) is used as, for example, a doping material for an AlGaInP-based compound semiconductor used for LEDs. As an example of a doping process, a process of introducing Cp2Mg into a process chamber together with carrier gas (e.g., hydrogen gas), and doping a semiconductor with magnesium produced by thermally decomposing Cp2Mg can be cited.
When performing this process, it is necessary to control the supply concentration of Cp2Mg. For this purpose, in the past, an acoustic type concentration measuring device such as an ultrasonic concentration meter, or an optical type concentration measuring device as disclosed in Patent Literature 1 has been used to measure the concentration of Cp2Mg in a supply pipe. In particular, in recent years, for some reasons such as the miniaturization of LEDs, a device capable of accurately measuring the concentration of Cp2Mg down to low concentrations has been required.
However, as a result of intensive examination made by the present inventor in consideration of such a requirement, it has been found that it is difficult for the conventional concentration measuring device to obtain sufficient measurement accuracy.
The reason for this is as follows.
It is said that Cp2Mg is a very stable material at temperatures lower than the temperature (approximately 300° C. to 500° C.) of thermal decomposition in a process chamber.
In addition, in the past, it has been considered that the inside of a supply pipe to the process chamber is kept at a temperature of, e.g., 30° C. to 50° C., which is far lower than the thermal decomposition temperature, and therefore inside the supply pipe, Cp2Mg is not decomposed but circulates in a stable state.
On the other hand, the present inventor has first found that the decomposition of Cp2Mg, which has been considered to be stable at low temperatures, occurs in an amount enough to cause a problem in highly accurate concentration measurement in a low concentration range although the amount is small.
Such decomposition results in the presence of a decomposed organic substance (such as methane) and magnesium in a supply pipe in addition to Cp2Mg in carrier gas (hydrogen).
As a result, the acoustic type concentration measuring device adapted to, on the assumption that only one material is present in carrier gas, measure the concentration of the material gives rise to an error in measured concentration in principle because of the presence of multiple materials produced by the decomposition.
On the other hand, in the past, the optical type concentration measuring device has measured the concentration of Cp2Mg on the basis of absorbance around 3.4 μm that is within the light absorption band of Cp2Mg. However, the light absorption band of the organic substance produced by the decomposition of Cp2Mg is also present around 3.4 μm, and therefore due to the effect of the organic substance, an error occurs in Cp2Mg measured concentration.